Famine reaction stops you losing weight-IT'S the reason most diets ultimately fail so Australian researchers are trying to crack the code of the "famine reaction".

They want to overcome the body's own defence system that forces it to combat weight loss by slowing down metabolism and increasing hunger when weight loss occurs.

Sydney University Associate Professor Amanda Salis is about to starve 100 obese women, allowing them only 800 calories a day so she can test whether drastic and rapid weight loss can defeat the famine reaction.

Her theory is that chemicals called ketones, released when the body's sugar stores are used up, will switch off the appetite increase that occurs after dieting and block a drop in the body's metabolic rate.

"We're looking at the role of ketones in reducing the famine reaction," says the professor who works at the Boden Institute of Obesity, Nutrition, Exercise & Eating Disorders

The principle she's testing is the same idea behind the popular new five and two Fast Diet pioneered by British journalist Michael Mosley.

The popular diet uses intermittent fasting (two days a week) to achieve weight loss.

"The idea is that by dieting for a short period of time, maybe the famine reaction doesn't get out of control," she said."We don't know yet what the optimum timing for being on a calorie restricted diet is."

Professor Salis has tested the theory on herself. She's lost 28 kilos and kept it off for over 15 years by using intermittent dieting.

She says she lost weight by repeatedly dieting for two weeks then having a 1-3 month period of weight maintenance in an attempt to beat the famine reaction.

Shocked by her own plan to impose a rapid weight loss regime on participants in her new medical trial she went on the 800 calorie a day diet for two weeks and lost four kilograms.

During the first four days of consuming just unappetising protein shakes and two cups of vegetables she felt hungry and headachy.

After that she no longer felt hungry and the weight dropped off fast, she said.

She warns the diet she is testing can only be undertaken under medical supervision.

The research is being funded by the National Health and Medical Research Council and Professor Salis is looking to recruit 100 women with a BMI of 30-40 who are menopausal to take part in the three-year trial.

She will give a public lecture on her research Wednesday night at the New Law School Foyer, University of Sydney.